1. Technical Field
The present invention is directed to acoustically investigating a borehole. More particularly, the present invention is directed to calculating the impedance of a material, e.g., cement or borehole fluid, behind a section of a casing located in a borehole. The impedance of the material behind the section of the casing is indicative of cement quality.
As used herein, "cement quality" refers to the qualitative determination of the presence and solidification of cement behind a section of casing located in a borehole. As used herein, cement quality is either "good," indicative of the presence of cement which has properly solidified, or "bad," indicative either of an absence of cement or its failure to solidify.
2. Background Information
A borehole is typically an 8 to 12 inch hole drilled or bored into the ground during the exploration of oil and/or gas reserves. Should a reserve be found, and should it be determined that the reserve would be profitable for production, the borehole is lined with casing, typically steel, and the casing is cemented to the borehole. The cement provides hydraulic isolation between the layers of the formation traversed by the borehole. The area(s) of production are accessed by perforating the casing at the requisite locations, and production of the reservoir is begun.
The borehole is cased for several practical reasons. For example, the borehole may have intersected several different reservoirs, such as water, gas, oil, or any combination of these. Casing the borehole and perforating only at those locations from which production is desired insures that there is no production of undesirable areas. Additionally, the casing insures that fluids from the desired areas of production are not lost to other areas within the formation.
In a typical case, a well is drilled in an area, commonly referred to as a field, where the depth and location of producing reserves are known. During the drilling process, casing is typically laid at regular intervals.
In order to insure hydraulic isolation between zones in the formation traversed by the borehole, the casing is cemented to the borehole. Without the cement seal, the fluids under pressure from one zone may flow through the annulus between the casing and the borehole to other zones. If the fluid is not the fluid of interest, the production zone could become contaminated to such an extent that production is not viable. Further, without the seal, the fluid of interest could escape from the zone determined for production, thereby rendering production uneconomical.
In preparing to cement a section of the casing to the borehole, a one-way check valve is inserted at the end of the casing to be cemented and a determination is made of how much cement will be needed. The cement is forced down the center of the casing, followed by a heavier solution, commonly brine or heavy mud, to insure that all of the cement is forced through the check valve. The cement travels through the valve, and eventually around the annulus between the casing and the borehole.
As the solution is heavier than the cement, all of the cement will have been forced through the check valve by the time the solution reaches the valve. The solution is then evacuated from the center of the casing, the valve removed, and this sequence of drilling and cementing continues until lining of the well is complete.
Defects in cement quality include annuli between the casing and cement. The annuli may destroy hydraulic isolation between zones. However, annuli less than 100 microns wide, commonly referred to as "micro-annuli", are generally able to preserve fluid isolation between zones, and are therefore not considered defects. Other types of defects include channels, as well as complete voids in the cement.
Defects in cement quality can also occur due to improper hardening of the cement. Field engineers estimate the amount of cement to be placed in the casing, based on borehole conditions. Cement powder is mixed with water to form the cement. If the water content in the formation about the borehole is not adequately taken into account, portions of the cement may not properly harden, due to an excess of water in the cement mixture.
Defects in cement quality can also occur during the life of the well. The installed casing may be exposed to various corrosions due to chemically active corrosive solutions, electrolytic corrosion due to ground currents, or contact with dissimilar metals. Corrosion of the outside casing wall may result in fluid communication between zones, an undesirable effect as explained above. Further, the corrosion may cause the casing to deteriorate to such an extent that the casing itself could collapse, also destroying the well. Once casing is installed in a well, it is difficult or impossible to remove the casing for above-ground inspection. Thus, it is imperative to be able to check the cement quality of the casing in situ.
Early devices for measuring the cement quality employed a sonde having a sonic transmitter spaced longitudinally from a receiver by a given distance, e.g., three feet. The transmitter was located below the receiver. The transmitter would produce a sonic pulse which would travel up through the casing to the receiver. The received signal was integrated over time, and based on the resulting integration, an indication of cement quality was obtained.
Theoretically, if the cement quality is good, the acoustical energy from the sonic pulse in the casing attenuates rapidly because it escapes through the cement seal and surrounding formation, due to the good mechanical coupling between the casing, cement and formation. Thus, the received signal should have a low amplitude. Conversely, if the cement quality is bad, the energy remains trapped in the casing and the received signal should have a relatively higher amplitude.
This device has several practical shortcomings. The spacing of the transmitter and receiver, typically 3 feet, inherently reduces the tool's resolution in the depth direction. Thus, it becomes difficult, if not impossible, to locate the true depth of the defect in cement quality. Additionally, as the tool is azimuthally symmetric, it is not possible to locate the relative azimuth of the defect in cement quality. Without such azimuthal information, it is not possible to determine what type of defect lies behind the casing.
Further, any tool eccentering, defined as a misalignment between the tool's center and the borehole's center, produces inaccurate cement quality determinations. Further still, due to the integration technique employed and the dependance on mechanical coupling to yield a cement quality determination, even slight micro-annuli which are adequate for sealing, but fail to provide good transfer of energy between casing/cement/formation interfaces, tend to produce false readings of poor cement quality.
The prior art has attempted to improve depth resolution by employing a transmitter/receiver pair having zero longitudinal offset, and has attempted to determine azimuthal resolution by effectively employing a plurality of transmitter/receiver pairs spaced about the perimeter of the tool. A prior art device which incorporates both features is shown, for example, in U.S. Pat. No. 4,255,798 issued to Havira, assigned to the same assignee as the present invention, and incorporated herein by reference.
Havira employs an acoustic pulse-echo technique having either a single transducer capable of directing its pulse at various azimuths or a plurality of transducers azimuthally located about the tool. Havira's technique for cement quality determination is dependent upon casing thickness. Thus, the transducer transmits a pulse having a frequency spectrum selected to stimulate the casing so as to produce a casing thickness resonance. The received signal includes an initial reflection segment, due largely to the reflection of the pulse off the casing's inner surface, and a reverberation segment, due largely to the subsequent reverberations from the resonating casing section. The reverberation segment is indicative of the energy of the echo produced by the casing-cement interface.
In Havira's preferred embodiment, the return waveform is amplified and rectified to obtain the d.c. signal representation of the amplitude of the waveform. This signal is filtered to obtain a signal representative of the envelope of the waveform. The circuitry processes the initial reflection and the reverberation segments separately. At the start of the return waveform, a pulse generator enables a gated amplifier, allowing the peak value of the initial reflection segment to be calculated and stored. The width of the pulse from the pulse generator is selected to enable the entire initial reflection segment to pass through the amplifier.
Thereafter, a second pulse generator enables a second gated amplifier, allowing the energy of the reverberation segment to be calculated by an integrator and stored. The width of the pulse from the second pulse generator selects a predetermined portion of the reverberation segment. The calculated reverberation segment energy is divided by the peak value to generate a normalized cement-bonding signal in order to remove the effects of tool tilt and borehole fluid anomalies.
The preferred frequency spectrum of Havira's acoustic pulse transmitter has a 6 dB bandwidth extending from about 275 kHz to about 625 kHz with a peak at about 425 kHz. This spectrum includes both the frequency of the fundamental thickness resonance for typical casing thicknesses, as well as higher order resonances in thicker casings. The received signal therefore includes the effects of both the fundamental and higher order resonances.
Havira overcomes many problems theretofore inherent in the prior art. For example, Havira is able to compensate for eccentering and the presence of micro-annuli. Additionally, Havira is able to resolve cement quality with a greater depth, as well as azimuthal, precision. However, wideband signal processing with fixed time windows leads to several inaccuracies in the determination of cement quality. For example, the wideband signal processing leads to inaccuracies due to the inclusion of unwanted noise components and phase variations between resonances. Additionally, processing with fixed time windows leads to inaccuracies due to the fact that the temporal portion of the reverberation segment signal indicative of cement bond varies with casing thickness.
The information indicative of cement quality is located in a narrow frequency band about the frequency of the casing's thickness resonance in the reverberation segment. By processing signals outside of this narrow band, wideband signal processing techniques include extraneous information which corrupts the cement quality determination.
Also, the resonances at different frequencies interfere with one another in a way which depends on the transducer, borehole fluid and other environmental effects. These effects, although unrelated to cement quality, nevertheless affect the signal which is used to determine cement quality.
Additionally, the segment of the reverberation segment containing information indicative of cement quality, e.g., bonding between the casing/cement interface, varies according to casing thickness. A thicker casing will produce a longer reverberation segment than will a thinner casing, given the same cement quality. These changes are not fully removed by the thickness normalization technique of Havira. Thus, by utilizing a fixed time window, the cement quality determination may not be correct.